The present invention relates generally to a control system for medium voltage variable frequency drives, and specifically to a control system for connecting multiple variable frequency drives together with a fiber optic communication ring.
In applications requiring multiple medium voltage variable frequency drives (“MV Drives”), there is typically a single drive designated as the main controlling drive. The remaining drives are then designated as followers. In such applications, typically the main controlling drive is responsible for the control of the overall speed (i.e., speed regulator), while the followers are responsible for assisting in providing the torque (i.e., torque regulators) to the motors in the system. In this arrangement, data is transferred from drive to drive over an industrial network such as, for example, DeviceNet, ControlNet or Industrial Ethernet. This network may also be shared with other controllers and drive combinations.
The main disadvantage experienced by such existing systems, is the creation of network latencies, that is, delays caused by network propagation delays and processing times in the system controllers and MV Drives. In critical applications, such as conveyors, such communication latency can create undesirable lateral or torsional vibrations. These vibrations can be amplified if they occur at certain natural frequencies. Another disadvantage of existing systems is that each MV Drive operates asynchronously from each other such that regulators add to the vibration in conveyor systems.
A secondary drive-to-drive communications link may be added to help alleviate the latencies of a single industrial network. However, such secondary links are typically over copper wire and therefore suffers from some of the same limitations as the industrial network such as lack of fault tolerance, excess latencies, limitations on distances, and the limitations of available bandwidth.
In the event of a break in communication with the controlling motor drives, either because of communication media failure or failure of the controlling drive itself, the system controller or PLC must we assign another drive to be the system controller or declare a fault in the system stopping all of the drives. Thus the motor drive system, even with a secondary drive-to-drive communication link, is susceptible to failure of (1) the PLC, (2) the main industrial control network, (3) the drive-to-drive communication link, and (4) the drive designated as the controlling drive.